If you post an example of the parts in question it would be easier to give a more specific answer.
If the component to be replaced is just a change in configuration, that can be done quite easily.
If it's a completely different shape, then you may have to simply delete the component to be replaced and insert the new one.
Could you show us what on some snapshots what do you want to do.
Here some examples what you must to consider:
If you want to...
If the Cast part is X and you want the Machined part to be Y just open Y and do an Insert Part and insert X there. This is a part to part relation.
In addition to the models requested above, what is your current workflow/procedure for this?
The reason I asked is that a company policy may dictate the workflow path.
Also, is the casting a separate part number from its machined state?
If so, you can then do as Steve Calvert suggested above and add your machining operations in the "Y" file.
I often use the Insert Part for exactly the purpose you mention, making a machined part from a cast part. The machined part will have new features, usually cuts, built on the casting. You cannot change the base part without destroying the machining's features built on the casting. For example, you have a cast part that is a 25mm cube. Insert that into another part and machine some holes in it. You cannot change the inserted part to something else without destroying all the machined holes.
However, you CAN use configurations of both the machined part and the cast part. This capability has probably been around for quite some time, but just this year I had need of it. I have a cast part file with several configurations. The machined part has the casting as its base part and then the machining features. I have several different machining configurations and some of the machining features are suppressed/unsuppressed and some dimensions are different between the configurations. Also the configuration of the base part casting is different between configurations. This is extremely powerful and made the overall work much less.
Perhaps using configurations of the base casting will serve your needs, but you cannot replace one base part with another.
In SWX there is always a way to do something and what you CAN do is insert multiple base parts, suppressing the ones you do not want active for the different configurations, though I would advise against this.
Another thought is whether you use (or plan to use) SolidWorks Costing.
Your workflow may need to be catered around this.
I don't use Costing, so it may be more workflow flexible than I'm implying.
Also, review the costing documents and tutorial as they show the before and after, whether it's from a block of stock or from a casting, which may give you more insight on your approach to this.
try "replace reference" in SW Explorer.
If your using insert parts into another part then you can not replace it once it's inserted. You can delete it and reinsert a different model, but anything you did after the insert would go as well. only way I think you could maybe replace it is to save the new model of the part you want to insert as the same name as the original inserted part and then close and re-open your part. But if it's to differn't from the orginal inserted part your going to have issues.